1. Field of the Invention
The present invention relates to cathode assemblies for use in Hall-Heroult aluminum reduction cells. Such cathode assemblies include a cathode block into which is fitted a collector bar. More particularly, the invention relates to cathode assemblies having multiple collector bars.
2. Description of the Related Art
Aluminum is commonly manufactured via a smelting process in an electrolytic cell of the established Hall-Heroult design. A conventional Hall-Heroult electrolytic cell, known as a pot and shown in FIG. 1, includes a cell C defining a chamber H in which are received carbonaceous anodes A. The anodes A are suspended in a bath B of electrolytic fluid containing alumina and other materials. Electric current is supplied to the anodes A via anode rod assemblies R to provide a source of electrons for reducing the alumina to aluminum which accumulates as a molten aluminum pad P. The molten aluminum pad P forms a liquid metal cathode. A cathode assembly CA, shown in detail in FIG. 2 from the underside thereof, is positioned in the bottom of the chamber H and completes the cathodic portion of the cell C. The cathode assembly CA includes a carbonaceous cathode block CB having an upper surface which supports the molten aluminum pad P and a lower surface which defines a groove or slot S extending between the ends of the cathode block CB. A collector bar BA, typically formed from hot rolled or cast mild steel, is received within the slot S and is secured in the slot S with a layer of a conductive material CM such as cast iron, carbonaceous glue, rammed carbonaceous paste or the like. The conductive material layer CM is disposed between the collector bar BA and the cathode block CB along the entire length of the slot S. The collector bar BA is longer than the cathode block CB and extends out of the chamber H. The exposed end of the collector bar BA is connected via a bus bar (not shown) to the current supply in a conventional manner to complete the circuit. The cathode assembly CA may include a pair of opposing collector bars BA as shown in FIG. 2 which are separated by a filler material F which fills the gap between the collector bars BA. The filler material F may be a crushable material or a piece of carbon or a carbonaceous paste, commonly referred to as seam mix or ramming paste (an unfired mixture of anthracite or graphite and anthracite and pitch binder), or a combination thereof
These electrolytic cells are typically operated at high temperatures (about 940 to 980.degree. C.) which, when combined with the corrosive nature of the electrolytes, creates a harsh environment. Collector bars conventionally are formed from hot rolled or cast mild steel. Mild steel has relatively poor conductivity compared to aluminum, but has a high melting point and relatively low cost. The cathode blocks have historically been formed from a mixture of anthracite and pitch binder and exhibit relatively high electrical resistivity, high sodium swelling, low thermal shock resistance and high abrasion resistance. As aluminum producers have sought to increase productivity, the operating amperages for such cells have been increased; hence the need for reduced power losses in the smelting process has increased. In an effort to reduce the electrical resistivity, graphite has been substituted for some of the anthracite in the cathode blocks, however with concomitant loss in abrasion resistance, increased erosion rates and higher cost of materials. Moreover, cathode blocks with high graphite content and cathode blocks that have undergone a graphitizing process are subject to uneven cathode current distribution along the length of the cathode block and high localized erosion rates.
An electrical current passing through an object naturally follows the path of least resistance. In the case of a Hall-Heroult cell, this is believed to be through the outer one-third of the cathode block CB. The lines D in FIG. 1 depict the uneven distribution of current passing through the cathode block CB and the high concentration of current passing through the outer one-third of the cathode block CB. This high concentration of current in the cathode block CB results in increased localized erosion rates in that portion of the cathode block CB.
Accordingly, a need remains for a device for and a method of improving the current distribution in cathode blocks of a Hall-Heroult electrolytic cell which permits high graphite content and graphitized cathode blocks to be operated at high amperage with an improved pot life expectation.